Subtropical Forests Act as Mercury Sinks but as Net Sources of Gaseous Elemental Mercury in South China

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China in 2014 to quantify Hg (gaseous elemental Hg, Hg 0, and reactive Hg, Hg II ) input and output fluxes and Hg retention in forests, consequently exploring the roles of subtropical forests in the global Hg cycle. At site Qianyanzhou, representing a background region with an enhanced atmospheric Hg 0 concentration, the total Hg II deposition (67.7 μg·m –2 ·year –1, 73% as dry Hg II deposition) was found to be slightly higher than the Hg 0 emission above the canopy (58.5 μg·m –2 ·year –1 ), indicating that the forest is a minor Hg sink but a significant net Hg 0 source on a yearly basis. In contrast, the forest in the moderately polluted region (site Huitong) acted as a significant Hg sink but a minor net Hg 0 source with a higher Hg II deposition (73.7 μg·m –2 ·year –1 ) and relatively negligible Hg 0 emission (2.65 μg·m –2 ·year –1 ). The decreasing atmospheric Hg 0 concentrations declined the total Hg sink based on the Hg budgets synthesized of this and previous studies and may promote forest Hg 0 emissions. Consequently, it was expected that the re-emission of historically deposited Hg may be enhanced from subtropical forests by recent decreases in atmospheric Hg 0 concentrations throughout China.